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System Data is the storage category macOS uses for everything that does not fit neatly into Photos, Documents, or Applications. It is not a folder you can open and empty. It is a label, and it covers caches, logs, snapshots, virtual machine images, and more.
What is inside
- App caches and temporary files that can be rebuilt.
- Local Time Machine snapshots, which macOS keeps for recovery.
- System logs and diagnostic reports.
- Large app support data, such as Docker images or Xcode device files.
- Mail downloads and Messages attachments.
What to do first
Open System Settings → General → Storage and wait for the categories to calculate. Click the info button next to System Data, then check Applications and Documents for unusually large items. A single virtual machine image can account for hundreds of gigabytes.
For caches and logs, a cleaner with a preview is safer than deleting folders by hand. Look for one that shows the size and location before anything moves. Avoid recipes that tell you to delete /System or /private folders from Terminal.
System Data is a symptom, not a villain. Find the large item, not a random folder.